1. Field of the Invention
The present invention is related to dynamometers and more particularly to electromechanical simulation of vehicle inertia and road load during vehicle testing on a dynamometer.
2. Brief Description of the Prior Art
Dynamometers are often used in vehicle testing where operation in place is desired. Because such test vehicles are not actually moving, means must be found to simulate certain parameters associated with the vehicle and its operation on the road. These parameters represent the forces required to accelerate the vehicle or sustain any speed.
Inertial forces, referred to herein as the parameter inertia, are a function of vehicle mass. Inertial forces must be overcome by the vehicle engine in order to achieve acceleration or deceleration. To sustain any speed, the vehicle engine must overcome forces which are related to the velocity of the vehicle. These forces, referred to herein as road load, include breakaway friction, rolling friction (as in bearings and tire of road friction), and windage (as in drag forces exerted on the vehicle by air). The purpose of the simulator is to impose forces upon the vehicle which represent those forces the vehicle would normally incur on the road. In the past, inertia simulation has been accomplished to some degree by adding a number of mechanically combinable flywheels to the dynamometer system. Road load is often simulated with hydrokinetic devices, pumps, electric brakes or electric motors. Most often multiple flywheels are used in conjunction with a road load simulating device. In some cases, an electric motor/generator is added to the system to provide electrically controlled loading and motoring. Sometimes a single flywheel and electric motor are used with the motor controlled to increase or decrease the inertia forces as determined by the operator.
Purely mechanical simulators (flywheel systems) provide accurate inertia simulation but are expensive. Changes must be made mechanically by clutching various flywheel combinations in and out of the mechanical train. Furthermore, the number of possible combinations is limited, depending on the number and sizes of wheels employed. Beyond this, the exact inertia is solely dependent upon the manufacturer's ability to calculate the moment of inertia of the flywheel, bearings, clutches, shafts and other components which rotate in the dynamometer. This is very difficult because of the various shapes, sizes and densities of these components. Accurate verification of the exact inertia is essentially impossible by the user, and no means of calibrating the inertia is available.
Prior art electromechanical inertia simulators are not considered as accurate as mechanical simulators. Present control systems are based on measurement of roll speed and subsequent computation of acceleration, and development of a torque control signal. Since this operation is essentially a differentiation of a parameter (velocity), which is changing slowly relative to the cause of the change--torque, there is a substantial time delay inherent in the system between roll speed change and inertia correction. Reported inertia response times (to 90 percent of value) are 0.5 seconds. There is also a settling time which ranges from 1 and 5 second depending upon differences between the desired inertia value and the inherent mechanical inertia of the dynamometer system upon the degree of change in vehicle torque.
In summary, presently available electromechanical dynamometer systems provide steady state inertia accuracies of the order of .+-.5 percent (vs .+-.1-3 percent for flywheel systems) and response times of the order of 0.5 seconds (vs negligible response time for flywheel systems). Duplication of road load forces is accomplished within .+-.3 percent (vs .+-.10-30 percent for hydrokinetic or brake type road load simulation).
At the present time, there is a need for more precise, accurate and fast response simulation of vehicle inertia and road load effects because of current U.S. Environmental Protection Agency rules defining test conditions during the monitoring of vehicle exhaust emissions. Further, the need exists to verify the accuracy of the inertia and road load simulation.